The shape of the lesson
By the end of the lesson, learners can
- Thin and broad: Two features of the whole leaf matter before you look inside.
- Inside a dicot leaf: Rotate the block cut from a leaf in the sim and click each tissue, or pick it from the list.
- Large surface area: absorbs more light. Thin: short diffusion distance for carbon dioxide.
- Top to bottom: cuticle, upper epidermis, palisade, spongy mesophyll with air spaces, lower epidermis with stomata.
- Vascular bundle: xylem brings water and ions; phloem takes sucrose away.
- Always link each structure to light, carbon dioxide, water or the products.
One picture every idea returns to
Most leaves have a large surface area, to absorb as much light as possible, and are thin, so carbon dioxide has only a short distance to diffuse to the mesophyll cells and light can reach all of them. Inside, palisade cells packed with chloroplasts sit near the upper surface, air spaces in the spongy mesophyll connect to the stomata for gas exchange, and veins bring water in the xylem and carry sucrose away in the phloem.
Forty-five minutes, phase by phase
| Time | Phase | What happens in the room | Grouping |
|---|---|---|---|
| 0 to 5 min | Hook: predict | Open with the question in the title, Built to catch light and swap gases.. A leaf is a food factory only a fraction of a millimetre thick. Put the figure on the board: it is the one picture every idea in the lesson returns to. | Whole class, then pairs |
| 5 to 17 min | Teach and check: Thin and broad | Two features of the whole leaf matter before you look inside. | Whole class, then pairs |
| 17 to 30 min | Teach and check: Inside a dicot leaf | Rotate the block cut from a leaf in the sim and click each tissue, or pick it from the list. Project tab 7 of the lab (3D leaf cross-section) and let a student drive it while the class predicts. | Think-Pair-Share |
| 30 to 36 min | Check | The four Stage 1 questions from the student lesson, answered on mini whiteboards or by show of fingers. Re-teach the one most get wrong. | Individual, then whole class |
| 36 to 43 min | Plenary | The exam-style questions from Stage 2, one at a time: thirty seconds alone, then a random call. The feedback lines in the key give the examiner's wording. | Individual, random call |
| 43 to 45 min | Exit and homework | Set the worksheet below (6 questions, 19 marks). Learners self-assess against the objectives before they leave. | Individual |
Protect the parts that carry the learning
The sequence carries about fifty minutes of material, so the lab is driven by one student while the class predicts rather than explored freely. Times are a guide.
Protected: the check at 30 minutes. It is the only individual read on whether the key idea landed, and it is not cut.
Cut first if the build overruns: the last check in Inside a dicot leaf, never the check or the plenary.
In a 60 minute block: run worksheet questions 1 to 3 in class as a Rally Coach set and model both the hardest worked example and its check live.
Think-Pair-Share
Pose the question, give thirty seconds of silent thinking, then a minute to compare with a partner before a few pairs share with the class. The silent minute is the part that cannot be cut.
The full structure, with its variations and the other twelve, is on the cooperative-learning page.
What to head off, and how
| Trap learners fall into | Teaching move that pre-empts it |
|---|---|
| Say what the feature does for photosynthesis: "large surface area to absorb more light", "thin, so a short diffusion distance for carbon dioxide". | Just writing "large surface area" scores nothing. |
| The upper epidermis cells have no chloroplasts, but guard cells do. | And in a vascular bundle in a leaf, the xylem is on the upper side and the phloem on the lower side. |
Leaf structure: worksheet
6 questions, 19 marks. Written in the style of the papers; no past paper question is reproduced.
State two external features of most leaves and explain how each is an adaptation for photosynthesis.
List, in order from the upper surface to the lower surface, the layers you would see in a section through a dicotyledonous leaf.
Explain how the palisade mesophyll is adapted for photosynthesis.
Describe the pathway taken by a molecule of carbon dioxide from the air to a chloroplast in a palisade cell, and explain how the leaf is adapted to make this quick.
A student counted stomata in the same area (0.1 mm2) of the upper and lower surfaces of five leaves. Means: upper surface 4, lower surface 26. (a) Calculate the mean number of stomata per mm2 on the lower surface. (b) Suggest an advantage of having most stomata on the lower surface.
A vascular bundle in a leaf contains xylem and phloem. Explain the role of each in photosynthesis, and identify which is on the upper side of the bundle.
Answer key and marking notes
- Large surface area, so more light can be absorbed (and more surface for gas exchange).
- Thin, so carbon dioxide has a short distance to diffuse to the mesophyll cells (and light reaches all the cells).
- Waxy cuticle, then upper epidermis.
- Palisade mesophyll.
- Spongy mesophyll with air spaces (vascular bundles run between the mesophyll layers).
- Lower epidermis with guard cells and stomata, then the lower cuticle.
- Palisade cells are close to the upper surface, where light intensity is highest.
- They contain many chloroplasts, with chlorophyll to absorb light.
- They are tall and closely packed, so many cells fit into the layer and absorb most of the light.
- Carbon dioxide diffuses in through an open stoma, controlled by guard cells.
- It diffuses through the air spaces of the spongy mesophyll.
- It dissolves in the moist surface of a mesophyll cell and diffuses into the cell and its chloroplast.
- The leaf is thin and the air spaces are connected, giving a short diffusion distance and a large internal surface area.
- (a) 26 ÷ 0.1 = 260 stomata per mm2.
- (b) The lower surface is shaded and cooler, so less water evaporates through the stomata.
- The upper surface is left free for light to reach the palisade cells, while gas exchange still happens through the many lower stomata.
- Xylem brings water, a raw material for photosynthesis, and mineral ions such as magnesium to the leaf.
- Phloem carries away sucrose made from the glucose produced in photosynthesis.
- In a leaf the xylem is on the upper side of the vascular bundle, with the phloem below it.
8 multiple-choice questions on this lesson
From the topic question bank. Assign them in the app for a marked score in your gradebook, or run the topic as a class game.
Which tissue in a leaf contains the most chloroplasts?
- Apalisade mesophyll
- Bspongy mesophyll
- Cupper epidermis
- Dxylem
What is the function of the xylem in a leaf?
- Ato bring water and mineral ions to the leaf
- Bto carry sucrose away from the leaf
- Cto let carbon dioxide into the leaf
- Dto absorb light for photosynthesis
Why is it an advantage that the waxy cuticle on the upper surface of a leaf is transparent?
- ALight can pass through to the palisade cells
- BCarbon dioxide can diffuse in through the wax
- CWater can evaporate quickly from the surface
- DOxygen can leave the leaf by this route
Which cells open and close the stomata of a leaf?
- Aguard cells
- Bpalisade cells
- Cphloem cells
- Droot hair cells
The diagram shows a simplified section through a dicotyledonous leaf. Which letter labels the spongy mesophyll?
- AC
- BB
- CD
- DE
Most leaves are thin. How does this increase the rate of photosynthesis?
- ACarbon dioxide diffuses a short distance to the cells
- BMore water evaporates to cool the leaf down
- CMore chloroplasts fit into each palisade cell
- DThe stomata stay open all through the night
Why are the air spaces in the spongy mesophyll important for photosynthesis?
- AThey give a large surface for carbon dioxide to diffuse into cells
- BThey store the oxygen made for use the next day
- CThey let light reach the lower epidermis cells
- DThey carry water from the xylem to every cell
A student counts 36 stomata in a field of view of area 0.12 mm2 on the lower surface of a leaf. What is the number of stomata per mm2?
- A300
- B4.3
- C30
- D3000
Quiz key
Palisade cells are tall, closely packed and full of chloroplasts, so most photosynthesis happens there. Spongy cells have fewer chloroplasts. Upper epidermis cells have none so light passes through, and xylem vessels are dead and carry water.
Xylem brings water, a raw material for photosynthesis, and mineral ions such as magnesium from the roots. Phloem carries sucrose away. Carbon dioxide enters through the stomata, and light is absorbed by chlorophyll in the chloroplasts.
A transparent cuticle lets light reach the palisade cells while the wax reduces water loss. The cuticle is waterproof and not the route for gases: carbon dioxide enters and oxygen leaves through the stomata, and the wax reduces, not speeds up, evaporation.
Each stoma is surrounded by a pair of guard cells that open and close it. Palisade cells are the main photosynthesising cells, phloem carries sucrose, and root hair cells are in the root, not the leaf.
C points to the rounded, loosely packed cells with air spaces between them: the spongy mesophyll. B is the tall palisade mesophyll, D is the vascular bundle (xylem and phloem), and E is a pair of guard cells around a stoma.
A thin leaf gives a short diffusion distance from the stomata to the mesophyll cells (and lets light reach every cell). Thinness does not add chloroplasts to a cell. Faster evaporation would lose water, and stomata usually close at night when there is no light for photosynthesis.
The connected air spaces let carbon dioxide from the stomata reach the moist surfaces of many mesophyll cells, a large internal surface area for diffusion. Oxygen is not stored there; it diffuses out. The lower epidermis needs no light, and water moves through the cells and walls, not the air spaces.
36 ÷ 0.12 = 300 stomata per mm2. 4.3 comes from multiplying 36 by 0.12 instead of dividing. 30 and 3000 come from misplacing the decimal point in 0.12.
What to project
The student lesson embeds these tabs. Open them full screen on the projector; every slider has a drag handle on the stage.
Slides for this lesson
A projectable deck for every pack is on the way. Until then, present the student lesson full screen: its sections, figure and lab tabs follow the same order as the plan above.